Techniques (pattern-forming techniques) in which a fine pattern is formed on top of a substrate, and a lower layer beneath that pattern is then fabricated by conducting etching with this pattern as a mask are widely used in the production of semiconductor devices and liquid display device. These types of fine patterns are usually formed from an organic material, and are formed, for example, using a lithography method or a nanoimprint method or the like. In lithography techniques, for example, a resist film composed of a resist material containing a base component such as a resin is formed on a support such as a substrate, and the resist film is subjected to selective exposure of radial rays such as light or electron beam, followed by development, thereby forming a resist pattern having a predetermined shape on the resist film. Using this resist pattern as a mask, a semiconductor or the like is produced by conducting a step in which the substrate is processed by etching.
The aforementioned resist material can be classified into positive types and negative types. Resist materials in which the exposed portions exhibit increased solubility in a developing solution is called a positive type, and a resist material in which the exposed portions exhibit decreased solubility in a developing solution is called a negative type.
In recent years, advances in lithography techniques have lead to rapid progress in the field of pattern miniaturization.
Typically, these miniaturization techniques involve shortening the wavelength (increasing the energy) of the exposure light source. Conventionally, ultraviolet radiation typified by g-line and i-line radiation has been used, but nowadays KrF excimer lasers and ArF excimer lasers are starting to be introduced in mass production. Furthermore, research is also being conducted into lithography techniques that use an exposure light source having a wavelength shorter (energy higher) than these excimer lasers, such as electron beam (EB), extreme ultraviolet radiation (EUV), and X ray.
As shortening the wavelength of the exposure light source progresses, it is required to improve various lithography properties of the resist material, such as sensitivity to the exposure light source and resolution capable of reproducing patterns of minute dimensions. As resist materials which satisfy such requirements, chemically amplified resists are known.
As a chemically amplified composition, a composition including a base material component that exhibits changed solubility in a developing solution under the action of acid and an acid generator component that generates acid upon exposure is generally used. For example, in the case where an alkali developing solution is used as a developing solution (alkali developing process), a base component which exhibits increased solubility in an alkali developing solution under action of acid is used.
Conventionally, a resin (base resin) is typically used as the base component of a chemically amplified resist composition. Resins that contain structural units derived from (meth)acrylate esters within the main chain (acrylic resins) are the mainstream as base resins for chemically amplified resist compositions that use ArF excimer laser lithography, as they exhibit excellent transparency in the vicinity of 193 nm.
Here, the term “(meth)acrylic acid” is a generic term that includes either or both of acrylic acid having a hydrogen atom bonded to the α-position and methacrylic acid having a methyl group bonded to the α-position. The term “(meth)acrylate ester” is a generic term that includes either or both of the acrylate ester having a hydrogen atom bonded to the α-position and the methacrylate ester having a methyl group bonded to the α-position. The term “(meth)acrylate” is a generic term that includes either or both of the acrylate having a hydrogen atom bonded to the α-position and the methacrylate having a methyl group bonded to the α-position.
In general, the base resin contains a plurality of structural units for improving lithography properties and the like. For example, a structural unit having a lactone structure and a structural unit having a polar group such as a hydroxy group are used, as well as a structural unit having an acid decomposable group which is decomposed by the action of an acid generated from the acid generator to form an alkali soluble group (for example, see Patent Document 1). When the base resin is an acrylic resin, as the acid decomposable group, in general, resins in which the carboxy group of (meth)acrylic acid or the like is protected with an acid dissociable group such as a tertiary alkyl group or an acetal group are used.
In a positive tone development process using a positive type, chemically amplified resist composition (i.e., a chemically amplified resist composition which exhibits increased alkali solubility in an alkali developing solution upon exposure) in combination with an alkali developing solution, as described above, the exposed portions of the resist film are dissolved and removed by an alkali developing solution to form a resist pattern. The positive tone process is advantageous over a negative tone development process in which a negative type, chemically amplified resist composition is used in combination with an alkali developing solution in that the structure of the photomask can be simplified, a satisfactory contrast for forming an image can be reliably obtained, and the characteristics of a resist pattern to be formed are excellent. For these reasons, currently, positive tone development process is tended to be mainly used in the formation of an extremely fine resist pattern.
As a lithography technique which has been recently proposed, a double patterning process is known in which patterning is conducted two or more times to form a resist pattern (for example, see Non-Patent Documents 1 and 2). According to the double patterning process, a resist pattern with a high level of resolution can be formed, as compared to the case where a resist pattern is formed by a single lithography step (namely, a single patterning process), even when a light source with the same exposure wavelength is used, or even when the same resist composition is used. Furthermore, double patterning process can be conducted using a conventional exposure apparatus. There are several different types of double patterning process, for example, (1) a method in which a lithography step (containing operation from application of resist compositions, exposure and developing) and an etching step are performed twice or more to form a pattern and (2) a method in which the lithography step is successively performed twice or more.
According to the double patterning process, for example, a first resist pattern is formed on a substrate by forming a resist film using a first resist composition and patterning the resist film, and then a resist film is formed using a second resist composition on the substrate on which the first resist pattern has been formed, followed by patterning of the resist film. As a result, a resist pattern can be formed with a higher level of resolution than that of the resist pattern formed through one single patterning process.
Furthermore, for example, by using SWT (side wall transfer) process in which an SiO2 film or an Si3N4 film is used as a sacrificial film and a mask is formed on both sides of the side walls of a pattern, patterning can be conducted with finer pitch than that of a resist pattern obtained by exposure and development of a resist film (see Patent Document 2). In this method, firstly a sacrificial film composed of an SiO2 film is subjected to etching and patterning using a resist pattern. Then, an Si3N4 film is formed on the SiO2 film pattern. Next, the Si3N4 film is subjected to etching such that the Si3N4 film remains only on a side wall portion of the SiO2 film. Thereafter, wet-etching is conducted to remove the SiO2 film, and the underlayer is subjected to etching using the remaining Si3N4 film as a mask.
However, when a double patterning process recited in non-Patent Documents 1 and 2 is applied, there is a problem that the number of steps is increased, and steps become complicated with high production cost, and productivity is decreased. In addition, the conventional SWT process as in Patent Document 2 needs wet-etching step, and therefore, both dry-etching and wet-etching are required. As a result, the SWT process is complicated. As a method of forming a pattern in simple process and low production cost, methods are disclosed in Patent Documents 3 and 4. Specifically, a method in which a photo resist film is subjected to exposure and development to obtain a first pattern, then an SiO2 film is formed on the obtained first pattern composed of a photo resist film, and next, the SiO2 film is subjected to etching such that the SiO2 film remains only on a side wall portion of the first pattern, and then the first pattern composed of a photo resist film is removed thereby forming a second pattern composed of the SiO2 film, is disclosed in Patent Documents 3 and 4.